Process for making liquid fuels from coal



Aug. 11, 1970 E} GQRlN ET AL Filed Feb. 24, 1969 2 Sheets-Sheet /o a I1 0 TO PRIMARY 4 HYDROCRAOKING zones 4 /00 20 /14 SOLVENT //6 x.

CARBONIZATION i '04 EFFLUENT H4 T0 CARBONIZATION ZONE FIG. 2

'INVENTORS United States Patent 3,523,886 PROCESS FOR MAKING LIQUID FUELS FROM COAL Everett Gorin, John A. Phinney, and Eric H. Reichl,

Pittsburgh, Pa., assignors to the United States of America as represented by the Secretary of the Interior, and Consolidation Coal Company, Library, Pa., jointly Filed Feb. 24, 1969, Ser. No. 801,626 Int. Cl. C10g 1/04 US. Cl. 208-8 3 Claims ABSTRACT OF THE DISCLOSURE In a process for making liquid fuel from coal by solvent extraction wherein the coal is extracted with solvent, and the extract is separated from undissolved coal residue by hydrocyclones, wherein the extract which is the overflow product from the hydrocyclone is catalytically hydrocracked to produce the desired liquid fuel, and wherein said undissolved residue which is the underflow product from the hydrocyclone is carbonized, the improvement comprising maintaining said underflow product as a pumpable slurry containing at least 35% by volume of liquid.

This invention relates to an improvement in a solvent extraction process for making liquid fuels from coal. In order to understand the present invention, it is necessary first to describe the solvent extraction process of which the invention is an improvement. That description is given below.

SOLVENT EXTRACTION PROCESS The solvent extraction process for the conversion of coal to synthetic liquid fuels, of which the present invention is an improvement, comprises a series of sequential, partial conversion steps, each of which is designed to effect most efiiciently the incremental addition of hydrogen or the progressive rejection of carbon, as the case may be. The basic process comprises:

( 1) Coal extraction;

(2) Separation of extract from undissolved coal residue;

(3) Primary catalytic hydrocracking of extract to produce high boiling distillable liquids;

(4) Catalytic hydrofining of the high boiling distillable liquids; and

(5) Secondary catalytic hydrocracking of the hydrofined high boiling distillable liquids to produce liquid fuels.

In the foregoing listed steps, progressive, incremental .addition of hydrogen is effected in the following generalized fashion. In the coal extraction step a small amount of hydrogen is added to the extraction zone either by the use of a hydrogen-transfer solvent or by the introduction of hydrogen gas or both. The purpose of this additional hydrogen is to permit the solvent extraction of up to 80 weight percent of the MAP (MAF means moisture-free and ash-free) coal. It is obviously desirable (for economic reasons) to recover a major portion of the coal as coal extract. However, coal as such has limited solubility in those solvents which it is practical to use in this process, unless hydrogen is added to partially upgrade the coal. While larger addition of hydrogen will permit greater depths of extraction, we have found that as the depth of extraction exceeds 80 weight percent, the hydrogen addition required to exceed such depths becomes economically prohibitive, as further explained hereinafter.

The products of extraction are separated in the second step to yield extract and undissolved coal residue. Most of the ash in the feed coal is recovered with the residue.

3,523,886 Patented Aug. 11, 1970 The extract is a solid at room temperature and contains very little (in general less than about 5 weight percent) material boiling below 400 C. The remainder of the extract is substantially non-distillable without decomposition.

It is highly desirable that the extract be free of ash before it or its upgraded products are introduced into the hydrogenation zones, particularly the latter two hydrogenation zones. The presence of such ash constituents, even in amounts as small as hundredths of one percent, seriously affects the activity and selectivity of the catalysts in the three hydrogenation zones. Deashing can be effected in the separation zone, in a distinctly separate deashing zone, in the primary hydrocracking zone, or in all three of these zones, as will be more full discussed later. For the moment, it is suflicient to point out that the ash that is left in the coal extract following separation from the residue is quite different in composition from that of the gross ash in the coal feedstock.

The function of the primary catalytic hydrocracking zone is to convert at least a portion of and preferably the major portion of the non-distillable coal extract to an ash-free, distillable hydrocarbonaceous liquid boiling below about 500 C. Generally the major portion of the distillable hydrocarbonaceous liquid boils in the range of 200 to 400 C. This partial upgrading treatment is carried out under relatively mild catalytic hydrogenation conditions which are particularly chosen so as to minimize gas and coke formation. These conditions, however, are not sufliciently severe to yield a distillable product which is free of nitrogen, oxygen, and sulfur (i.e., N-O-S) compounds, or to yield a distillable product, the major portion of which boils below about 200 C., that is, in the gasoline boiling range.

The function of the catalytic hydrofining zone is to remove substantially all of the N-O-S contaminants from the ash-free, distillable hydrocarbonaceous liquid fed thereto. If desired, all of the distillable hydrocarbonaceous liquid obtained from the primary hydrocracking zone may be introduced into the hydrofining zone; preferably, however, only the fraction boiling below about 400 C. is introduced therein. It is important to note that the hydrofining treatment is designed primarily to remove N-O-S contaminants from the feed material and is not designed to effect any major lowering of the boiling range of the feed material. For example, the major portion of the N-O-S-free efiluent hydrofiner products still boils above about 200 C., and generally in the range of 200 to 400 C.

The function of the secondary catalytic hydrocracking zone is to lower the boiling range of at least the higher boiling fraction of the effluent hydrofiner products. This final, partial upgrading treatment is carried out under hydrogenation conditions and in the presence of an eflicient cracking catalyst especially suited for such purpose. The selection of the optimum cracking catalyst in the secondary catalytic hydrocracking zone is permitted because of the previous removal of ash contaminants and N-O-S contaminants and because of the previous conversion of the original non-distillable extract to substantially non-coking, distillable hydrocarbonaceous liquid. In general, the liquid fuel product from the secondary hydr0 cracking zone boils below 200 C., that is in the gasoline boiling range.

The use of the terms primary and secondary is merely for convenience of reference and does not mean that the secondary hydrocracking zone is subordinate to the primary hydrocracking zone.

THE PRESENT INVENTION The present invention resides in an improvement in the operation of the above-described separation step of the solvent extraction process. Instead of substantially completely separating the extra from undissolved residue, only a partial separation is elfected to permit the recovery of the residue in the form of a fiowable slurry. In order to obtain a flowable slurry, at least 35 volume percent of the slurry must be liquid at the operating temperature. We have found that such a slurry may be obtained as the underflow from a hydrocyclone suitably operated to produce the desired result. We have also found that the overflow from such operation of a hydrocyclone will contain more than 95 percent liquid including both extract and solvent, and even more importantly, that the solids in such extract overflow are no more harmful to the life of the catalyst in the primary hydrocracking zone than the solids present in extract obtained by substantially complete separation of extract and residue. Thus, the problems associated with the complete separation of liquid extract from the solid residue are avoided. The underflow slurry may be subjected to low temperature carbonization in much the same manner as the dry solids. The increased solids content of the overflow is allowed to build up in the hydrocracking zone, to a point where it is rejected to a coking zone for recovery of any carbon value. The final yield of distillable hydrocarbonaceous liquids is not significantly less than that achieved when complete separation of extract and residue is effected. A lower cost and more operable process is thus provided. In fact, by using a two-stage arrangement of the hydrocyclones incorporating a washing step, as will be shown, the overall recovery of extract is 95 percent, which is equal to or better than is achieved in the case of complete separation of extract and residue.

For a better and more complete understanding of our invention, its objects and advantages, reference should be had to the following description and to the accompanying drawing which is a diagrammatic illustration of the preferred embodment of the present invention.

PREFERRED EMBODIMENT The following, with reference to the drawing, is a description of the preferred embodiment of the present invention. The preferred embodiment comprises:

(1) A solvent extraction zone wherein the coal is extracted;

(2) A separation zone consisting essentially of hydrocyclones wherein the extract is partially separated from the residue;

(3) A carbonization zone 26 wherein the underflow from the hydrocyclones is carbonized to produce a liquid distillate and a solid hydrocarbonaceous solid product, referred to as char;

(4) A deashing zone 40 wherein at least a portion of the ash in the extract is removed;

(5) Three primary catalytic hydrocracking zones 50 wherein the extract is converted to distillable hydrocarbonaceous liquid;

(6) A coking zone 68 wherein a portion of the unconverted extract from the primary hydrocracking zones is coked to produce coke and a liquid distillate;

(7) A hydrofining zone 74 wherein N-O-S compounds are removed from distillable hydrocarbonaceous liquid; and

(8) A secondary catalytic hydrocracking zone 84 wherein high boiling N-O-S-free efi'luent hydrofiner products are converted to gasoline.

FEED COAL Any coal may be used in the process of our invention, non-limiting examples of which are lignite, bituminous coal, and sub-bituminous coal. Preferably, the coal fed to our process is one having a volatile matter content of at least 20 weight percent, for example, a high volatile bituminous coal such as Pittsburgh Seam coal. A typical composition of a Pittsburgh Seam coal suitable for use in the process of our invention is shown in Table I.

4 TABLE I Proximate analysis: Wt. percent MF 1 coal Volatile matter 39.3 Fixed carbon z 47.7 Ash 13.0

' I 100.0 Ultimate analysis: Wt. percent MAF coal Hydrogen 5.5 Carbon 80.8 Nitrogen 1.4 Oxygen 7.5. Sulfur 4.8 We? 1 ME means moisture-free.

The feed coal is preferably ground to a finely divided state, for example, minus 14 mesh Tyler Standard screen, and is freed of substantially all extraneous water before introduction into the process.

SOLVENT EXTRACTION ZONE Coal is introduced into a solvent extraction zone 10 via a conduit 12. Fresh hydrocarbonaceous solvent and recycle solvent are introduced into the extraction zone 10, via conduits 14 and 25', respectively. The coal and the solvent react therein to yield the desired coal extract.

The solvent extraction process may be any of the processes commonly used by those skilled in the art, e.g., continuous, batch, countercurrent, or staged extraction, at a temperature in the range of 300 to 500 C., a pressure in the range of 1 to 6500 p.s.i.g., a residence time in the range of 1 to 120 minutes, a solvent to coal ratio of 1/1 to 4/1, and, if desired, in the presence of a catalyst and/or up to 50 standard cubic feet of hydrogen per pound of MAF coal.

Suitable solvents for the coal in the extraction step are polycyclic, aromatic hydrocarbons which are liquid under the temperature and pressure of extraction. Preferably, at least a portion of the aromatics are partially or com pletely hydrogenated. Mixtures of the above hydrocarbons are generally, used and are derived from intermediate or final steps of the process of this invention, for example, from the primary hydrocracking zone products or from the hydrofining zone products. Examples ofsuitable solvents are tetrahydronaphthalene, decalin, biphenyl, methylnaphthalene, and dimethylnaphthalene. Other types of coal solvent such as oxygenated aromatic compounds may be added to the above-mentioned types for special reasons, for example, to improve the solvent characteristics, but the resulting mixture should be predominantly of the types mentioned. Examples of additive oxygenated solvents are the phenolic compounds such as phenol, cresols, and xylenols.

A particularly preferred solvent is a portion of the product obtained from the primary catalytic hydrocracking zone. This solvent normally comprises a 325 to 425 C. fraction blended with some lower boiling material.

The coal and the solvent are maintained in intimate contact within the extraction zone 10 until the solvent has extracted, i.e., converted or dissolved, up to weight percent of the MAF feed coal. As previously mentioned,

in order to attain the above depths of extraction, hy-.

drogen must be added to the coal during extraction. We prefer to add the hydrogen by the use of a hydrogen transfer solvent of the types described above.

We have found that at least 50 weight percent of the MAF coal must be extracted in order to attain economic extract yields. We have further found thatif more than 80 weight percent of the MAF coal is extracted, the cost associated with non-selective transfer of hydrogen that takes place at those depths becomes prohibitive. Thus, the amount of hydrogen which is added to the coal during extraction is only that amount which is necessary to accomplish the desired coal extraction, i.e., to dissolve up to 80 weight percent and preferably to dissolve between 50 and 80 weight percent of the MAP coal.

SEPARATION ZONE Following extraction, the mixture of solvent, extract, and residue is conducted rapidly, so as to avoid cooling of the mixture, through a conduit 18 to a separation zone 20, consisting essentially of hydrocyclones. The hydrocyclones are adapted to operate at elevated temperature, for example 600 F. and at elevated pressures. While such hydrocyclones may be operated so as to effect substantially complete separation of liquid and solids, the improvement accomplished by the present invention requires operation of the hydrocyclone to assure an underflow which is a flowable slurry. We have found that at least 35 volume percent of the underflow slurry must be liquid. Only sufficient liquid should be present in the slurry to assure fiowability, since more than that unnecessarily diverts extract from transfer to the hydrocracking zone. The overflow from the hydrocyclone is transferred by a conduit 22 to a solvent recovery zone 21 where solvent is removed by distillation and some of it is recycled through conduit to the extraction zone 10. The balance of the solvent is recycled through conduit 23 to the conduit 18 for readmission to the hydrocyclone. Such recycle to the hydrocyclone helps to control the amount of extract contained in the underflow slurry from the hydrocyclone. Loss of extract to the carbonization zone may be reduced by using a two-stage arrangement of the hydrocyclones which incorporates a solvent washing step. In such arrangement, two hydrocyclones are connected so that the underflow slurry from the first is introduced into the second hydrocyclone as its feed. Recycle solvent from the solvent recovery zone 21 is introduced into the conduit connecting the two hydrocyclones to dilute the feed to the second hydrocyclone. This solvent removes some of the extract from the second hydrocyclone as part of the overflow, thus reducing the amount of extract in the underflow, which goes to the carbonization zone. The overflow from the second hydrocyclone may be returned to join the feed to the first hydrocyclone. Another arrangement, involving three hydrocyclones, is shown in FIG. 2 and is described more fully below.

The following table tabulates the results of a test program utilizing a 3-inch liquid cyclone at 600 F. and 150 p.s.i.g. on extraction eflluent from extraction zone 10.

Coal feed rate, lbs/hr 1,300 1,300 2,080 Extraction solvent/coal ratio 4. 7 3. 0 1. 5 Cyclone feed:

Percent solids 6. 9 9. 9 16.1 Percent extract in solvent 10.7 15. 8 27. 1 Liquid vise, cp 0.35 0.48 1.1 Liquid specific gravity 0. 743 0. 754 0. 780 Feed rate, g.p.m 18.8 12.7 12.0 Pressure drop, p.s.i 122 109 96 Cyclone performance:

Solids in overflow, wt. percent 0.3 1.0 3. 3 Solids in underflow, wt. percent 35. 5 40. 4 34. 8 Underilow, wt. percent feed 18. 8 22. 4 38. 7 Solids in solvent-free extract, wt

percent 2. 7 6. O 11.1 Percent extract to overflow. 86. 8 85. 0 68. 5 Percent solids to underflow... 96. 7 92. 4 87. 9 Estimated D50, microns 7. 5 13. 0 17.0

CARBONIZATION ZONE The overflow from the hydrocyclones is introduced via the conduit 24 into a low temperature carbonization zone 26. I

The carbonization zone 26 is maintained at a temperature in the range of 425 to 760 C. Preferably, the Zone 26 is a fluidized low temperature carbonization zone; however, if desired, other conventional devolatilization zones may be used, e.g. a rotary kiln. Hydrocarbonaceous solids, i.e. char. are withdrawn from the zone 26 via a conduit 28, while a liquid distillate is withdrawn via a conduit 30. Preferably, the liquid distillate of carbonization is separated in a fractionation zone 32, to yield:

(1) A fraction boiling below 230 C. (withdrawn via a conduit 34) which is treated in a conventional tar acid plant (not shown) to recover phenol, cresols, and xylenols;

(2) A fraction boiling between 230 and 325 C. (withdrawn via conduit 36) which is introduced into the hydrofining zone as subsequently explained; and

(3) A bottom fraction boiling above 325 C. (withdrawn via a conduit 38) which is admixed with the feed to the hydrocyclones 20. This bottom fraction generally contains some finely divided solids which are separated from the liquid portion by such recycle to the hydrocyclones.

DEASHING ZONE Returning to the recovered extract (the extract obtained from extract recovery zone 21), the extract, prior to hydrogenation in the primary catalytic hydrocracking zone, is preferably treated in a deashing zone 40 to remove any ash contained therein which is harmful to hydrocracking catalysts. These ash constituents that remain in the extract may be removed, at least in part by chemical treatment, for example, with acids.

PRIMARY HYDROCRACKING ZONE Extract, which contains solids unaffected by the deashing treatment, is introduced via a conduit 48 into the first of -a series of staged, dense bed, liquid phase fluidized catalytic hydrocracking zones 50. For convenience purposes, three primary hydrocracking zones are shown; however, if desired, any number may be used.

The extract, which as previously mentioned is substantially non-distillable without decomposition, is reacted with hydrogen in the presence of the fluidized catalyst in the primary hydrocracking zones 50 under the following conditions:

Reactor temperature4l0 to 475 C.

Reactor pressure (total pressure)-2500 to 6000 p.s.i.g.

Hydrogen feed rate2000 to 42,000 s.c.f./bbl. feed Liquid hourly space velocity (individual stage)--0.5 to

3.0 volume/volume/hour Vaporous products, which are solids-free, are withdrawn from the zones 50 via the conduits 52 and conveyed via a common conduit 54 to a condenser (not shown) wherein non-condensable gases are separately recovered. The condensed vaporous product, i.e., the solidsfree, distillable hydrocarbonaceous liquid product, is then introduced into a fractionation zone 56, wherein it is fractionated to yield:

(1) A fraction boiling below 260 C. (withdrawn via a conduit 58) which is subsequently catalytically hydrofined;

(2) A fraction boiling between 260 and 325 C. (withdrawn via a conduit 60), the major portion of which is subsequently catalytically hydrofined; and

(3) A fraction boiling above 325 C. (withdrawn via the conduit 14) which is introduced into the extraction zone 10 as fresh solvent.

Preferably, a portion of the 260 to 325 C. fraction is conveyed via a conduit 62 and introduced into the extraction zone 10 along with the +325 C. fraction, which usually boils below about 500 C. In some instances, it may be desirable to further fractionate the +325 C. fraction such that only the 325 to 425 C. frac tion is used as extraction solvent while the +425 C. bottoms are recycled to the primary hydrocracking zones 50 or to a coking zone as hereinafter discussed. Obviously, many other variations in the above fractionation of the distillable hydrocarbonaceous liquid may be practiced by those skilled in the art. For example, all of the distillable liquid product may be hydrofined, in which case fresh extraction solvent would be recovered from the hydrofiner products.

The non-distillable extract fed to the primary hydrocracking zones is only subjected to sufficient hydrocracking therein to yield a distillable liquid product suitable for subsequent hydrofining. The distillable hydrocarbonaceous liquid product is not completely free of N-O-S compounds, nor does a significant portion thereof boil in the gasoline boiling range, i.e. boil below about 200 C. Generally, the major portion of the distillable hydrocarbonaceous liquid boils in the range of 200 to 400 C.

Returning to the primary hydrocracking zones, the non vaporized extract is withdrawn from each of the zones 50 (via a conduit 64) and then introduced into the following zones in succession. If desired, however, rather than inroduce all of the unconverted extract into the next primary hydrocracking zone, a portion may be recycled to aid in maintaining the hydrocracking catalyst in a fluidized bed. The non-vaporized extract from the last primary hydrocracking zone is preferably recycled to the same zone.

A portion of the recycled unconverted extract is coked, e.g. in a delayed coker to yield a liquid distillate and coke. As shown in the drawing, a portion of the recycled unconverted extract, together with the solids that have been carried through from the hydrocyclones, is conveyed via a conduit 66 into any conventional type coking zone 68. The carbonaceous solids from the hydrocyclones are of such a fine size that they readily pass through the catalyst beds. Liquid distillate is recovered from the coking zone 68 via a conduit 70 and conveniently fractionated with the liquid distillate of carbonization in the fractionation zone 32. Coke is withdrawn from the coking zone 68 via a conduit 72.

Coal extract can be hydrogenated in the presence of an active, supported, regenerable catalyst, specifically a catalyst comprising a mixture of a group 6B oxide or sulfide with a group 8 metal. The use of such a supported catalyst to hydrogenate the extract in the primary hydrocracking zone makes possible the conversion of the extract to the distillable hydrocarbonaceous liquid product at mild conditions of temperature and pressure. Other catalysts, for example, molten zinc chloride, may also be used.

HYDROFINING ZONE Broad Preferred Temperature 340 to 470 C 380 to 430 C. Pressure (total pressure) 500 to 4,500 p.s.i.g 1,000 to 3,000

p.s.1.g.

Hydrogen ratio 1,000 to 10,000 1,500 to 3,000

s.e.f./bbl. feed. s.e.f./bbl. feed. Liquid hourly space velocity 0.2 to 2.0 volume] 0.5 to 1.5 volume/ volume/hour. volume/hour.

The efiluent hydrofiner products, recovered from the zone 74 via a conduit 76, are substantially free of nitrogen, oxygen, and sulfur com ounds. The efiluent products are fractionated in a fractio ation zone 78 into a gasoline fraction boiling below about 193 C. (recovered via a conduit 80) and a secondary hydrocracker zone feedstock boiling above about 193 C. (recovered via a conduit 82).

The hydrofining catalyst may be disposed in a fixed stationary bed, or various moving bed or fluidized bed techniques may be used. Generally, the fixed bed technique, which is illustrated on the drawing, is most satisfactory. Suitable catalysts may comprise any of the oxides or sulfides of the transitional metals, and especially an oxide or sulfide of a group '8 metal (preferably iron, cobalt, or nickel) mixed with an oxide or sulfide of a group 6B metal (preferably molybdenum or tungsten). Such catalysts may be used in undiluted form, but normally are supported on an adsorbent carrier such as SECONDARY HYDROCRACKING ZONE At least the higher boiling portion of the efiluent hydrofiner products are introduced via the conduit 82- into a secondary catalytic hydrocracking zone 84. The hydrofiner products are reacted therein in the presence of a catalyst with hydrogen under the following conditions to produce additional gasoline boiling below about 193C. (recovered via a conduit 86).

Broad Preferred Reactor temperature 340 to 500 C 380 to 440 C. Reactor pressure (total 500 to 4,500 p.s.i.g 1,200 to 3,000

pressure). p.s.i.g.

Hydrogen ratio 2,000 to 10,000 3,000 to 6,000

s.c.f./bbl. feed. s.e.f./bbl. feed. Liquid hourly space velocity. 0.3 to 3.0 volume/ 0.5 to 1.5 volume/ volume/hour. volume/hour.

Since the nitrogen, sulfur, and oxygen compounds present in the distillable hydrocarbonaceous liquid recovered from primary hydrocracking zone 50 are removedin the hydrofining zone 74, the hydrocracker feedstock is preferably reacted with hydrogen in the secondary hydrocracking zone 84 in the presence of an active cracking catalyst which also exhibits some hydrogenation activity. Suitable catalysts include a mixture of a transition group metal such as cobalt and an oxide of a group 6B metal such as molybdenum on an acid support such as silicaalumina. Platinum acidic oxide catalysts, for example, those which include between about 0.05 and 2.0 weight percent of the catalyst of at least one metal of the platinum conia, silica-alumina-thoria, alumina-boria, silica-magnesia, silica-alumina-magnesia, silica-aluminadluorine and the like. A preferred support is a synthetic composite of silica and alumina.

Referring to FIG. 2 of the drawings, there is shown schematically a hydrocyclone system including two hydrocyclones and 102 whose primary function is separation of extract and solids and a third hydrocyclone 104 whose primary function is washing of the combined underflow streams from the other two hydrocyclones. Extract eflluent from the solvent extraction zone 10 (see FIG. 1 is introduced by a conduit 106 into the first hydrocyclone 100 where the separation of the extract eflluent into an overflow product slurry of low solids concentration and a pumpable underfiow product slurry of high solids concentration is effected. The overflow product is conducted by a conduit 108 to the second hydrocyclone 102 where a further separation is effected. The overflow product from this hydrocyclone 102 may contain up to five percent solids which are of much smaller size consistency than the catalyst particles used in the primary hydrocracking zones to which the overflow product is conducted by a conduit 110. The fine solids in the overflow product readily pass through the catalyst in the hydrocracking zone, and as pointed out above, do not deleteriously affect the catalyst life any more than extract which has been separated from residue by filtration, despite the relatively large concentration of solids in the extract from the hydrocyclone system operated to effect only such partial separation.

The underflow product from hydrocyclone 102 is conducted by a conduit 112 to the underflow conduit 114 from hydrocyclone 100. Solvent is introduced into conduit 114 by a conduit 116. This solvent corresponds to that shown in FIG. 1 as being recycled through conduit 23. Eflluent from the carbonization zone (designated 26 in FIG. 1) is introduced through a conduit 118 into conduit 114. The combine-d underflow streams, solvent and carbonization effluent are introduced into the third hydrocyclone 104 for a final wash and recovery of more extract. The overflow from this hydrocyclone is conducted by a conduit 120 to the overflow line 108 from the first hydrocyclone. The underflow from hydrocyclone 104 is withdrawn through a conduit 122 to the carbonization zone 26 (see FIG. 1).

What we claim is:

1. In a solvent extraction process for making liquid fuels from coal which includes the steps of (a) coal extraction, (b) separation of extract from undissolved coal residue, (c) carbonization of the coal residue, and (d) catalytic hydrocracking of the extract, the improvement providing for a system of pum-pable slurries which comprises passing the slurry of extract and undissolved coal residue from the coal extraction step through a hydrocyclone system having at least one hydrocyclone to effect only partial separation of liquid and solids to yield an overflow product of low solids concentration slurry and a pumpable product underflow of high solids concentration slurry, said underflow slurry containing at least 35 percent by volume of liquid, subjecting the low solids concentration slurry to the aforesaid catalytic hydrocracking step, and subjecting the high solids concentration slurry to the aforesaid carbonization step.

2. A process according to claim 1 wherein the hydrocyclone system includes at least two hydrocyclones interconnected so that the underflow slurry from the first is introduced into the second as its feed after dilution with solvent.

3. A process according to claim 1 wherein the hydrocyclone system includes at least three hydrocyclones interconnected so that the overflow slurry from the first is introduced into the second as its feed, and the underflow from both the first and second hydrocyclones is introduced into the third hydrocyclone as its feed after dilution with solvent.

References Cited UNITED STATES PATENTS 3,143,489 8/1964 Gorin 208--8.

DELBERT E. GANTZ, Primary Examiner V. OKEEFE, Assistant Examiner 

